function [t, v] = skydiver_if
% Function to calculate skydiver falling speed with
% input method selected on the fly.
%
% Inputs:  mass m (kg)
%          gravity g (m/s^2)
%          drag coefficient c (kg/m)
%          time vector (s)
%
% Outputs: falling speed vector v (m/s)

% Check if user wants to input parameter values interactively.
% If not, assume the user will input parameter values from a file.
% Use am if statement for this example.

response = input('Do you want to enter parameter values interactively? (y or n): ','s');

if strcmp(response,'y') | strcmp(response,'Y')
    % Read parameter value inputs
    m = input('Enter mass in kg: ')
    g = input('Enter gravity in m/s^2: ')
    c = input('Enter drag coefficient in kg/m: ')
    t = input('Enter time vector in s: ');

elseif strcmp(response,'n') | strcmp(response,'N')
    % Read filename
    filename = input('Enter the name of the data file containing parameter values: ','s');

    % Verify that a filename was entered
    if isempty(filename)
        fprintf('No file name entered - aborting program.\n')
        return
    end

    data = load(filename);
    nrows = size(data,1);

    m = data(1,1)
    g = data(2,1)
    c = data(3,1)
    t = data(4:nrows,1);
        
else
    % Bad input - return to Matlab command prompt
    fprintf('Illegal input - please rerun the program and enter y or n.\n')
    t = [];
    v = [];
    return
end

v = sqrt(m*g/c)*tanh(sqrt(g*c/m)*t);

% Determine if terminal velocity was reached
% This is the most typical use of an if statement
tol = 1e-4;

npts = size(v,1);
dv = abs(v(npts,1)-v(npts-1,1));

if dv < tol
    disp('Terminal velocity was researched.')
else
    disp('Terminal velocity was not reached.')
end